As an introduction to problems solved by the present invention, consider the conventional magnetic tape drive used for storing information from a personal computer. Such a tape drive records data on magnetic tape spooled inside a removable tape cartridge. The tape drive includes a read/write head and a transport mechanism for moving the tape across the head. Magnetic tape generally includes dozens of tracks arranged either parallel to a center line along the length of the tape or arranged in a helical fashion at an angle to the center line of the tape.
The conventional head is a laminated structure that includes a tiny gap for each track. The gap for a particular track is more narrow in width than the width of the track to allow for some misalignment of the tape on the head without the gap departing from the particular track. Misalignment is corrected by moving the head across the width of the tape so that the gap returns to the center line of the particular track. When the head is moved, all gaps are realigned. Therefore as the tape is transported across the head, a gap for reading data is continuously aligned over a data track when a second gap is maintained by servo control over a second track containing prerecorded information that indicates the extent of misalignment.
Conventional track following servo systems position a read head by determining position information from a signal read by the head from a servo track. Reliable head positioning is made difficult by variation in the quality of the signal. Such variation is attributable to many sources including, for example, defects in the tape, defects in the manner in which the servo track was prerecorded, defects and dimensional changes in the tape including stretching that occurred since the servo track was prerecorded, debris between the servo read head and the tape, variation in tape tension and speed across the read head, and noise sources common to electromechanical systems.
In conventional servo systems, these difficulties are met in part by employing complex prerecorded information at the expense of tape capacity. Servo system accuracy limitations reduce the maximum safe data recording density, having similar undesirable effects on tape capacity and seek time. Attempts to increase servo system stability and positioning accuracy have led to higher tape speed requirements so that more media is available for more complex servo signal patterns. Use of conventional servo system design techniques in applications having low tape speed results in low tape capacity and weak reliability.
As a consequence of such limitations, further improvement of tape system responsiveness at a computer system level is impeded. Tape systems serve personal backup, as well as institutional services. The limitations described above have economic impact on virtually everyone who operates a tape library, provides database management services, or operates a computer with an expectation of reliable data storage and quick retrieval.
In view of the problems described above and related problems that consequently become apparent to those skilled in the applicable arts, the need remains in track following servo systems for improved circuits and methods for determining position information from a signal read from tape and for improved methods and apparatus for prerecording such signals.